Hot-forming steel materials, hot-forming members, and methods for manufacturing the same.

Abstract

The present invention provides hot-forming steel materials, hot-forming members, and methods for manufacturing them, which have high strength, excellent surface quality, and fatigue characteristics. [Solution] The invention provides a hot-forming steel material comprising a base steel sheet and a plating layer formed thereon, wherein the base steel sheet contains, by weight %, C: 0.04~0.45%, Si: 1.5% or less, Mn: 0.2~2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities, and satisfies the carbon concentration index before hot forming: C(peak, before HPF) / C(nom, before HPF) ≥ 1.5. A method for manufacturing the hot-forming steel material is also provided, which includes the step of box annealing the above-mentioned plated steel sheet.

Description

[Technical Field]

[0001] This invention relates to hot-forming steel materials, hot-forming members, and the manufacturing of the same, used in automobiles and the like. Regarding the method. [Background technology]

[0002] Recently, efforts have been made to improve fuel efficiency by making cars lighter. To achieve this, the thickness of the steel materials is being reduced. While this is effective, reducing the thickness may cause problems with the vehicle's stability. Because of this possibility, the strength of the steel material must be improved. For this reason, high strength There is a continuing demand for high-grade steel plates, and various types of steel materials are being developed. However, Steel materials like these have high strength, but this also means they have poor workability.

[0003] To solve these problems, hot forming or This is called Hot Press Forming (HPF). A hot forming method has been proposed. The hot forming method involves processing the steel material at a high temperature, which makes it easier to process, By rapidly cooling this to a low temperature, a low-temperature structure such as martensite is formed within the steel material. This is a method to increase the strength of the final product. By this method, a component with high strength can be produced. This minimizes the processing challenges involved in manufacturing.

[0004] Patent document 1 is an example of a technology related to such hot forming. Patent document 1 describes Al-S After heating the plated steel sheet to over 850°C, the components are assembled by hot forming using a press and rapid cooling. By forming the weave as martensite, ultra-high strength with a tensile strength of 1600 MPa or more is achieved. We propose a technology to secure it. In the case of the technology proposed in Patent Document 1, molding is done at high temperature. Complex shapes can be easily molded, and the strength is increased by rapid cooling within the mold. It is expected to have a weight-reducing effect.

[0005] On the other hand, hot-formed components used for passenger protection require excellent durability, and typical Excellent fatigue characteristics are required as an indicator. For example, the B-pillar (B-pi) of an automobile. In cases like llar, repeated stress and deformation cycles over a long period of time Fracture can occur under stresses much lower than the yield strength or tensile strength, and in certain cases... The material needs to have the ability to support loads exceeding 100kcal without breaking (fatigue properties).

[0006] Fatigue properties, the main factor determining the durability of a material, have historically been used primarily in bearings and vehicle shears. This has been considered a required property for steel used in applications such as steel. However, recently, high strength of hot-formed steel has been a key factor. With the change in temperature, hot-formable steel materials or parts thereof applied to the pillars of automobiles The importance of fatigue properties is constantly increasing in materials as well.

[0007] Various methods have been proposed to improve the above fatigue characteristics. For example, in Patent Document 2 This involves ensuring surface hardness through heat treatment after the surface carburizing and nitriding treatment of the product, thereby improving the durability of the parts. This document presents a method to improve the product surface. Patent Document 3 describes shot peening ( By applying a shot peening treatment to form compressive residual stress on the surface, the material This paper proposes methods to improve the fatigue lifespan of materials.

[0008] However, the above-mentioned methods are applied after the product has been molded, and plating and other processes are not performed. When this occurs, it may inhibit the surface quality, so there is a limit to applying the above method. There is a limit that it cannot be done.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] The problem of the present invention is that the hot-formed member has high strength, excellent surface quality and fatigue characteristics, a hot-formed steel material having the same, a hot-formed member manufactured using the same, and a method for manufacturing them. It is to provide.

[0011] The problems of the present invention are not limited to the above matters. Further problems of the present invention are described in the entire specific content of the specification, and for those having ordinary knowledge in the technical field to which the present invention belongs anyone can understand further problems of the present invention from the content described in the specification of the present invention without any difficulty.

Means for Solving the Problems

[0012] One embodiment of the present invention includes a base steel plate and a plating layer formed on the base steel plate, wherein the base steel plate contains, by weight %, C: 0.04 to 0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2 to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0 ​.01~0.1%, Cr:0.01~5.0%, N:0.02% or less, balance Fe and not allowed Contains impurities, This relates to a hot-forming steel material that satisfies the following [Relationship Formula 1].

[0013] [Relationship 1] Carbon concentration index before hot forming: C (peak、HPF前) / C (nom、HPF前) ≥1. 5 (In relational equation 1, C (peak、HPF前) This is at a point on the surface where the plating layer thickness is 1 / 3 of the way through. From the direction of the base steel sheet as described above, the first carbon particle that appears in the carbon profile, according to the GDS analysis results, is Peak is the highest carbon value, C (nom、HPF前) is Steel Nominal (N (This is the carbon value.)

[0014] Another embodiment of the present invention is one in which, by weight, C: 0.04-0.45%, Si: 1.5% or less Below (excluding 0%), Mn: 0.2-2.5%, P: 0.05% or less, S: 0.02% or less , Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, balance F A step of obtaining a plated steel sheet using a steel slab containing e and unavoidable impurities, The above plated steel sheet is box annealed in a one-step manner as described in [Box Annealing Condition 1] below, and the hot annealing is performed in a step including This relates to a method for manufacturing steel materials for forming.

[0015] [Box annealing condition 1] Temperature range (T1): 500~800℃ Retention time (t1): 1 minute or more (this is the retention time at the target temperature) Heating rate (H1): 20-160°C / hr (This is the rate at which the temperature rises to the target temperature.)

[0016] Yet another embodiment of the present invention is a mixture of C: 0.04-0.45% and Si: 1% by weight. Less than 5% (excluding 0%), Mn: 0.2-2.5%, P: 0.05% or less, S: 0.02 % or less, Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, A step of obtaining a plated steel sheet using a steel slab containing residual Fe and unavoidable impurities, The above plated steel sheet is box annealed in a two-stage method as described in [Box Annealing Condition 2] below, and the hot annealing is performed in a two-stage method. This relates to a method for manufacturing steel materials for forming.

[0017] [Box annealing condition 2] Temperature range for one interval (T2-1): 500~780℃ Holding time for one interval (t2-1): 1 minute or more (this is the holding time at the target temperature). Heating rate per section (H2-1): 20~160°C / hr (This is the heating rate to reach the target temperature.) ) Temperature range for two sections (T2-2): 600~800℃ Holding time for the two sections (t2-2): 50 minutes or more (this is the holding time at the target temperature). Heating rate in two sections (H2-2): 0.25~160℃ / hr (at the heating rate up to the target temperature) be)

[0018] Yet another embodiment of the present invention is a mixture of C: 0.04-0.45% and Si: 1% by weight. Less than 5% (excluding 0%), Mn: 0.2-2.5%, P: 0.05% or less, S: 0.02 % or less, Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, The base material comprises the remainder Fe and unavoidable impurities, and a plating layer formed on the base material. This relates to a hot-formed member that satisfies the conditions of [Relational Equation 2] below.

[0019] [Relationship 2] C (peak、HPF後) / C (nom、HPF後) ≥0.1 (In relational equation 2, C (peak、HPF後) This is at a point on the surface where the plating layer thickness is 1 / 3 of the way through. From the direction of the base steel sheet, the first carbon peak that appears in the carbon profile, according to the GDS analysis results. (Peak) is the highest carbon value, C (nom、HPF後) Nominal of steel (Nom (This is the carbon value.)

[0020] Another embodiment of the present invention is the step of obtaining a blank using the hot-forming steel material described above. , The above blank is heated to a temperature of Ac3-980°C and then held for 1-1000 seconds. and, Hot forming, including the step of hot forming the heated and held blank and then cooling it. This relates to a method for manufacturing components. [Effects of the Invention]

[0021] According to the present invention, the material has high strength after hot forming, as well as excellent surface quality and fatigue properties. It is possible to manufacture hot-formed members with excellent durability. Hot-formed steel for this purpose It is possible to provide materials, hot-formed members manufactured therefrom, and methods for manufacturing them. Cut.

[0022] The diverse and beneficial advantages and effects of the present invention are not limited to those described above, but include specific aspects of the present invention. This can be more easily understood in the process of describing the embodiments. [Brief explanation of the drawing]

[0023] [Figure 1] This graph shows the cross-sectional structure of the steel (a), the GDS (Glow Discharge Spectrometer) analysis results performed to measure the Cpeak value before and after hot forming, and the carbon profile (b). [Figure 2] (a) and (b) are TEM images showing the cross-sections of Invention Example 1 and Comparative Example 1 after box annealing heat treatment, respectively. [Figure 3] (a) and (b) are optical microscope images of the surfaces of Invention Example 2 and Comparative Example 7 after box annealing heat treatment, respectively. [Modes for carrying out the invention]

[0024] The terms used herein are for illustrative purposes only and do not limit the present invention. This is not intended. Furthermore, the singular form as used herein is used in a manner that is clearly contrary to the relevant definition. Unless otherwise specified, this includes multiple forms.

[0025] As used herein, "includes" means to embody a configuration and the presence or addition of other configurations. This does not mean exclusion.

[0026] Unless otherwise defined, all terms used herein, including technical and scientific terms. This is the same meaning as that generally understood by a person with ordinary skill in the art to which this invention belongs. It has meaning. The terms defined in the dictionary correspond to the relevant technical documents and currently disclosed content. It is interpreted as having the meaning of doing so.

[0027] First, one embodiment of the hot-forming steel material of the present invention will be described in detail. In weight percent, C: 0.04~0.45%, Si: 1.5% or less (excluding 0%), Mn: 0 .2~2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~0.1% Contains Cr: 0.01-5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities. This is possible. Below, we will explain each alloy composition in detail, where % means weight percent. .

[0028] Carbon (C): 0.04~0.45% The above C is an essential element added to improve the strength of the component. If the ratio is less than 0.04%, it becomes difficult to ensure sufficient strength, and even if the flexibility is ultimately high, collisions can occur. Since energy absorption capacity actually decreases, adding 0.04% or more is effective. On the other hand, when the carbon content exceeds 0.45%, although the strength increases, the flexibility decreases, and impact resistance... Since energy absorption capacity decreases, a concentration of 0.45% or less is effective.

[0029] Silicon (Si): 1.5% or less (excluding 0%) The above-mentioned Si should be added as a deoxidizing agent in steelmaking, and furthermore, as a solid solution strengthening element. It is also an element that suppresses carbide formation, contributing to increased strength of hot-formed components and effectively improving material uniformity. It is added as an element. If its content exceeds 1.5%, it will be added to the surface of the steel sheet during annealing. The resulting Si oxide may reduce the plating quality. Therefore, the above Si is It is effective if it is included at a concentration of 1.5% or less (excluding 0%).

[0030] Manganese (Mn): 0.2-2.5% The above Mn not only ensures a solid solution strengthening effect, but also improves curing ability, It is necessary to add it to suppress ferrite formation during hot forming. The above Mn content If the concentration is less than 0.2%, there are limitations in achieving the above effects, and the insufficient curing ability needs to be improved. This leads to the problem that other expensive alloying elements are required in excess, significantly increasing manufacturing costs. Problems may arise. On the other hand, if the above Mn exceeds 2.5%, the steel sheet The increased strength may reduce cold rolling properties, and the microstructure phases are aligned in the rolling direction. The band structure may become more pronounced, potentially reducing its ability to absorb collision energy. Therefore, an effective Mn content is 0.2 to 2.5%.

[0031] Phosphorus (P): 0.05% or less The above P exists as an impurity in the steel, and if its content exceeds 0.05%, hot working This may significantly weaken the weldability of the molded component. On the other hand, the above P is an impediment during steel manufacturing. It is an impurity to be avoided, and there is no particular limit on its lower limit, but the P content should be 0.001%. Controlling it to less than 0.001% may require high manufacturing costs, therefore, it should be 0.001% or higher. That's fine.

[0032] Sulfur (S): 0.02% or less The above-mentioned S exists as an impurity in the steel and hinders the ductility, impact properties, and weldability of the hot-formed member. Since it is a harmful element, it is effective to limit it to a maximum of 0.02%. On the other hand, the above S is An unavoidable impurity, and there is no need to specifically limit its lower limit, but 0.0001% or less Achieving perfect control may require high manufacturing costs, therefore, a tolerance of 0.0001% or more is acceptable. That's fine.

[0033] Aluminum (Al): 0.01-0.1% The above-mentioned Al, along with Si, is an element that enhances the cleanliness of steel by performing a deoxidizing effect in steelmaking. If the above Al content is less than 0.01%, the above effect is difficult to obtain, and if the content is 0.1% If it exceeds this, the high-temperature ductility decreases due to the excess AlN formed during the continuous casting process, causing slag. There is a problem with crack formation. Therefore, the Al content mentioned above should be 0.01~0 A concentration of 0.1% is effective.

[0034] Chromium (Cr): 0.01-5.0% The above Cr, like Mn, is used to ensure the hardening ability of steel and to ensure a beautiful surface during the HPF process. It is added. If the above Cr content is less than 0.01%, it becomes difficult to ensure sufficient curing ability. It is possible. On the other hand, if the content exceeds 5.0%, the effect of improving curing ability is relative to the amount added. The amount is small, which promotes the formation of coarse Cr-based carbides and reduces the collision energy absorption capacity. Because of the possibility of over-exposure, it is effective to keep the percentage below 5.0%.

[0035] Nitrogen (N): 0.02% or less The above N is present in the steel as an impurity. If the above N content exceeds 0.02%, as previously stated... Similar to the case of Al, the formation of AlN makes slab cracks more likely to occur. There is a question. The above N is an impurity, and there is no particular limit on its lower limit, but N contains Controlling the quantity to less than 0.001% may require high manufacturing costs, It may be 0.001% or more.

[0036] On the other hand, the above steel material contains, in addition to the alloy components mentioned above, Mo: 0.5% or less, Ni: 0.5% or less. Below, one or more of the following are included: Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less. It can be included in these.

[0037] Molybdenum (Mo): 0.5% or less The above-mentioned Mo, like Cr and Mn, not only has the effect of improving the hardening ability of steel, but This allows for effects such as increased flexibility due to grain refinement resulting from the formation of fine precipitates. However, if the above Mo content exceeds 0.5%, the cost increase of ferroalloys becomes excessive compared to the effect. Therefore, it is effective for its content not to exceed 0.5%. The above Mo content is A level of 0.45% or less is more effective, and a level of 0.4% or less is even more effective. Therefore, a level of 0.35% or less is even more effective.

[0038] Nickel (Ni): 0.5% or less The above Ni is an austenite-stabilizing element, and the addition of Ni improves the hardening ability of steel. It can be done. However, since Ni is an expensive alloying element, the manufacturing process is affected by the hardening ability improvement effect. Considering the increase in manufacturing costs, setting the upper limit at 0.5% would be effective. On the other hand, N To fully obtain the curing ability improvement effect from the addition of i, it is necessary to include at least 0.01% i. It is effective, and a concentration of 0.03% or higher is more effective, and 0.05% or higher is also effective. This is even more effective. The upper limit of Ni above is more effective when it is 0.45%, and 0 A concentration of 0.4% is even more effective, while 0.35% is the most effective.

[0039] Niobium (Nb): 0.1% or less The above Nb is an element that can obtain a precipitation-enhancing effect by forming fine precipitates, This results in increased strength and improved flexibility due to refined crystal grains. Furthermore, during heating for hot forming, excessive grain growth is suppressed, and fluctuations in heat treatment conditions are controlled. This can improve robustness against [unspecified factor]. However, if the above Nb content exceeds 0.1%, the effect will be reduced. Not only does the precipitation process become saturated, but the increasing precipitation temperature also leads to an increase in relatively coarse precipitates, and costs Efficiency may be reduced compared to [another method]. Therefore, the above Nb content is 0.1% or less. This is effective. The lower limit of the above Nb content is effective when it is 0.005%, and 0 A percentage of 0.01% is more effective, and 0.015% is even more effective. 。It is more effective that the upper limit of the above Nb content is 0.09%, and it is even more effective that it is 0.08%, and it is most effective that it is 0.07%. and it is even more effective that it is 0.08%, and it is most effective that it is 0.07%.

[0040] Titanium (Ti): 0.1% or less The above Ti may be added in combination when adding B to ensure hardenability by combining with nitrogen remaining as an impurity in steel to form TiN. It is an element that is also added in some cases. In addition, due to the formation of TiC precipitates, precipitation strengthening and grain refinement effects can be expected. However, when the Ti content exceeds 0.1%, rather coarse TiN is formed in large quantities, deteriorating the impact energy absorbing ability. Therefore, it is effective that the upper limit is 0.1%. The lower limit of the above Ti is effective that it is 0.005%, more effective that it is 0.01%, and even more effective that it is 0.015%. It is more effective that the upper limit of the above Ti is 0.08%, even more effective that it is 0.06%, and most effective that it is 0.05%. and it is most effective that it is 0.05%.

[0041] Boron (B): 0.01% or less The above B can not only improve hardenability even with a small addition, but also segregate at the boundaries of the old austenite grain boundaries, effectively suppressing the brittleness of the hot-formed member due to grain boundary segregation of P and / or S. [[ID=3�]]However, when its content exceeds 0.01%, it causes brittleness in hot rolling due to the formation of Fe 23 CB6 composite compounds. Therefore, it is effective that the upper limit is 0.01%. On the other hand, the lower limit of the above B content is effective that it is 0.0001%, more effective that it is 0.0003%, and even more effective that it is 0.0005%. and it is even more effective that it is 0.0005%. Furthermore, it is effective. The upper limit of the above B content is 0.009%, which is more effective. A rate of 0.007% is even more effective, and 0.005% is the most effective. ru.

[0042] The remainder contains iron (Fe), and is an unintended impurity from the raw materials or surrounding environment during the normal manufacturing process. These impurities may inevitably be present, and therefore cannot be eliminated. This is something that any ordinary technician in the manufacturing process would understand, therefore, in this specification It does not specifically mention all of its contents.

[0043] The above hot-forming steel material includes a plating layer on at least one surface. The above plating layer is made of zinc (Z n) Plating layer, aluminum (Al) plating layer, etc., the type is particularly limited. Furthermore, there are no particular limitations on the method of forming the plating layer, such as hot-dip plating or electroplating. As a typical example, an Al-based plating layer may be formed. The above Al-based plating is not limited to... Although not fixed, as an example, the above Al-based plating layer has Si: 6-12% by weight, Fe: 1-4% are formed after plating with a plating bath containing Al and unavoidable impurities. Therefore, the plating layer may be an alloyed plating layer formed by subsequent box annealing. That is, The plating layer in the above steel material is considered to be a pure plating layer plated by a plating bath. This is difficult to explain, and it is preferable to consider the plating layer as an alloy formed between the base steel sheet and the plating layer.

[0044] The above hot-forming steel material has a surface carbide concentration index before hot forming, as defined by [Relationship Formula 1] below. (Surface carbon segregation factor) is 1.5 or more. Being superior is effective.

[0045] [Relationship 1] C (peak、HPF前) / C (nom、HPF前) ≥1.5 (In relational equation 1, C (peak、HPF前) This is at a point on the surface where the plating layer thickness is 1 / 3 of the way through. From the direction of the base steel sheet, the first carbon peak that appears in the carbon profile, according to the GDS analysis results. (Peak) is the highest carbon value, C (nom、HPF前) Nominal of Steel (minal) Indicates the carbon value, and usually refers to the target carbon content within the steel.

[0046] The inventors of this invention applied box annealing to a steel sheet for hot forming and observed the surface properties after box annealing. Furthermore, after observing the surface properties, fatigue properties, etc. of the member after hot forming, the following results were obtained: box annealing conditions and after box annealing. When controlling the surface properties of steel plates to a consistent level, it is necessary not only to ensure the surface quality of hot-formed components, We have come to recognize that it is possible to improve fatigue characteristics.

[0047] Specifically, in order to improve the fatigue characteristics in order to enhance the durability of the hot-formed member described above, In addition, the surface carbide concentration index of hot forming steel materials defined by the above [Relationship Equation 1] (Surface The carbon segregation factor (CFO) being 1.5 or higher is effective. It is effective.

[0048] The microstructure of the above hot-forming steel material contains 50-90% ferrite and 3% pearlite. One of the following: 0% area or less, bainite 20% area or less, and martensite 20% area or less. It can contain more than one.

[0049] The above ferrite is a soft phase, and reduces the load during the blank blanking process of the steel material. It is an effective organization for reduction, and for that purpose, it is effective if it covers 50% or more of the area. However, If the area exceeds 90%, excessive carbon is present in structures other than ferrite during blank preparation. The carbon may be distributed unevenly even after hot forming. Therefore, the above-mentioned fer Lighting is most effective when it covers 50-90% of the area.

[0050] If the above-mentioned perlite exceeds 30% area, cementite will not dissolve completely after hot forming. This can lead to a decrease in strength or cause material inconsistencies. On the other hand, If inite or martensite each exceeds 20% area, the strength of the steel plate becomes excessive. The rise in temperature could lead to problems such as mold wear during blank production.

[0051] On the other hand, the whiteness of the above hot-forming steel material may be 60 or higher. Plating layer of hot-forming steel material If the heat treatment of box annealing for alloying (pre-alloying) is performed excessively, the surface oxide will If excessive formation occurs, the whiteness may be poor and may fall below 60. If the surface quality deteriorates, Since problems such as roll contamination may occur during hot forming, the box annealing conditions described later are performed. This is effective.

[0052] Next, one embodiment of the method for manufacturing hot-forming steel materials according to the present invention will be described in detail. The manufacturing method described above is only one of all possible embodiments, and the hot process This means that forming steel should not necessarily be manufactured only by the following manufacturing methods. There isn't one.

[0053] A plated steel sheet is manufactured using a steel slab that satisfies the alloy composition described above, and the plated steel sheet obtained is the same as above. The steel plate is box-annealed.

[0054] The surface carbon concentration index of the steel material before hot forming, as defined in [Relationship Equation 1] above, Alternatively, the surface carbon concentration index of the hot-formed member, as defined in [Relationship Formula 2] described later, must be 0.1 or higher. To ensure this, box annealing can be performed. The above box annealing is performed in one step. It can be carried out in p) or two steps, and the box annealing conditions can be varied accordingly. Let's do that. We will explain each of these in detail, divided into [Box Annealing Condition 1] and [Box Annealing Condition 2].

[0055] [Box annealing condition 1] Temperature range (T1): 500~800℃ Retention time (t1): 1 minute or more (this is the retention time at the target temperature) Heating rate (H1): 20-160°C / hr (This is the rate at which the temperature rises to the target temperature.)

[0056] If the above heating rate (H1) is less than 20°C / hr, the heat treatment time will be longer. Surface quality deteriorates due to oxide formation on the surface, and when the heating rate (H1) exceeds 160°C / hr Due to overheating of the edge portion caused by temperature deviation in the width direction, the plating layer melts. This may result in a decrease in surface quality. The target temperature range (T1) is less than 500°C. When box annealing is performed at this temperature, not only is alloying of the plating layer poor, but the surface carbon concentration index is also affected. Defects may result in inferior surface quality and fatigue properties. On the other hand, if the temperature exceeds 800°C... Excessive alloying may degrade surface quality. Also, the holding time at the target temperature may be affected. A holding time of 1 minute or more is effective; if the holding time is less than 1 minute, the entire plating layer will be alloyed. There is a risk that the alloying will not occur. However, if the holding time exceeds 100 hours, excessive alloying may occur. This may result in a decrease in surface quality.

[0057] [Box annealing condition 2] Temperature range for one interval (T2-1): 500~780℃ Holding time for one interval (t2-1): 1 minute or more (this is the holding time at the target temperature). Heating rate per section (H2-1): 20~160°C / hr (This is the heating rate to reach the target temperature.) ) Temperature range for two sections (T2-2): 600~800℃ Holding time for the two sections (t2-2): 50 minutes or more (this is the holding time at the target temperature). Heating rate in two sections (H2-2): 0.25~160℃ / hr (at the heating rate up to the target temperature) be)

[0058] If the heating rate in the above section is less than 20°C / hr, the heat treatment time will be excessive, and the surface The quality may deteriorate, and melting may occur at the edges if the temperature exceeds 160°C / hr. Surface quality may deteriorate when the heating rate in the two sections is less than 0.25°C / hr. Excessive heat treatment can lead to an increase in surface oxides, potentially degrading surface quality. When the temperature exceeds 0°C / hr, the surface quality deteriorates due to melting in the plating layer at the edges. It's possible.

[0059] If the holding time at the target temperature for the above section is less than 1 minute, the surface carbon concentration index is met. It is difficult. However, there is no particular upper limit, but if the retention time exceeds 100 hours, Excessive alloying can potentially degrade surface quality and excessively increase process costs. Problems can occur. On the other hand, if the holding time at the target temperature in the two sections is less than 50 minutes. It is difficult to satisfy the surface carbon concentration index. However, there is no particular upper limit, but the retention time is 1 If the process time exceeds 0 hours, the process cost will increase excessively, and surface oxides will increase, affecting surface quality. There is a risk that it may decrease.

[0060] On the other hand, the target temperatures for the above sections 1 and 2 must not exceed 780°C and 800°C, respectively. This is effective. If the above target temperature is exceeded, the surface whiteness due to over-alloying will be less than 60. Surface quality may deteriorate, and the increased heat input could lead to excessively high process costs. There is a possibility that this will happen. On the other hand, the target temperatures for section 1 and section 2 are not yet 500°C and 600°C, respectively. In full, it is necessary to maintain a heat treatment of more than 100 hours for complete alloying during box annealing. The decrease in surface quality and the increase in heat input may lead to excessive process costs. The target temperature for this section is higher than the target temperature for the entire section.

[0061] The purge gas in the above-mentioned box annealing furnace is hydrogen (H2), nitrogen (N2), and a mixture thereof. One of these is effective, and the purge amount is 0.1 to 100. m 3 A rate of / hr is effective. The above purge amount is 0.1m 3 If it is less than / hr, The atmosphere inside the box annealing furnace is not controlled, which may lead to a decrease in surface quality, and the purge amount is 100. m 3 If the temperature exceeds [number] hours, the amount of heat supplied to maintain the temperature inside the furnace increases, and the manufacturing cost increases. It could become excessive.

[0062] Furthermore, the circulation fan can be activated during box annealing, and the amount of the circulation fan's operation at this time is A rotation speed of 10 rpm or higher is effective. The above-mentioned circulation fan operates at a low speed of 10 rpm. If the temperature is less than this, melting will occur due to temperature deviations in the width direction of the coil and overheating of the edges. Surface quality may deteriorate. The higher the operating rate of the above circulation fan, the better. This is determined according to the capacity of the circulating fan motor, so there is no particular upper limit. Undetermined.

[0063] There are various methods for manufacturing the above-mentioned plated steel sheet, and as an example, the composition range described above The steel slabs that meet the requirements are subjected to processes such as heating, hot rolling, coiling, cooling, cold rolling, annealing, and plating. It can be obtained through the following process. The following explains each step.

[0064] Heating of steel slabs The above steel slab is heated to 1050-1300°C. Below ℃, not only is it difficult to homogenize the structure of the steel slab, but it also makes it difficult to utilize the precipitated elements. Furthermore, it may become difficult to re-solidify the mixture. On the other hand, if the heating temperature exceeds 1300°C, excess A thick oxide layer may form, increasing the likelihood of inducing surface defects after hot rolling. Therefore, Therefore, the heating temperature of the steel slab mentioned above is effective at 1050-1300°C. The lower limit of the heating temperature for the lubricant is 1070°C, which is more effective than 1100°C. This is even more effective. The upper limit of the steel slab heating temperature mentioned above is 1280°C, which is more effective. It is a target, and 1250°C is even more effective.

[0065] Hot rolling The heated steel slab described above is hot-rolled and then finish-hot-rolled at 800-950°C to produce hot-rolled steel. A sheet is obtained. If the above finishing hot rolling temperature is less than 800°C, the surface layer of the steel sheet due to two-phase rolling A mixed grain structure may develop in the area, making it difficult to control the plate shape. On the other hand, the above hot finishing process The question is whether grain coarsening due to hot rolling is likely to occur when the rolling temperature exceeds 950°C. There is a problem. Therefore, the above-mentioned finishing hot rolling temperature of 800-950°C is effective. Therefore, the lower limit of the above-mentioned finishing hot rolling temperature is more effective at 810°C, and 82 0°C is even more effective. The upper limit for the above-mentioned finish hot rolling temperature is 940°C. This is more effective, and 930°C is even more effective.

[0066] winding The above hot-rolled steel sheet is wound at 500-700°C. If the above winding temperature is less than 500°C Furthermore, martensite is formed on the entire or partial surface of the steel plate, making it difficult to control the plate shape. The increased strength of the hot-rolled steel sheet leads to a decrease in its rollability during subsequent cold-rolling processes. This can occur. On the other hand, if the winding temperature exceeds 700°C, coarse carbides will form. Therefore, the collision energy absorption capacity of the hot-formed member may decrease. An effective winding temperature is 500-700°C. The lower limit of the winding temperature is 520°C. °C is more effective, and 550°C is even more effective. A maximum temperature of 680°C is more effective, and 650°C is even more effective. It is effective.

[0067] cooling The hot-rolled steel sheet, once wound, is cooled from the winding temperature to 400°C at a rate of 10°C / hr or more. Cooling is performed at a certain temperature (hot rolling cooling), but if the above cooling rate is less than 10°C / hr, carbides will form. A defect is that a large number of coarse carbides are formed during the cooling of hot-rolled coils due to the long cooling time. Spots may occur. Therefore, the above cooling rate should be 10°C / hr or higher for it to be effective. It is effective, and it is more effective at temperatures above 12°C / hr, and above 15°C / hr. This is even more effective. On the other hand, as long as the above cooling rate is 10°C / hr or higher, this Since the desired effect can be obtained through this process, there is no particular upper limit.

[0068] On the other hand, a pickling step can be added after the above cooling and before cold rolling. This removes scale formed on the surface of the steel plate, improving the surface quality of the product. It can be made to happen.

[0069] cold rolling After the above process, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio during the cold rolling process is Therefore, although not specifically limited, a reduction ratio of 30-80% is required to obtain the target steel thickness. This can be applied.

[0070] Annealing and Cooling The above cold-rolled steel sheet is annealed. To do this, the above cold-rolled steel sheet is heated, and at this time 4 Heating at a rate of 20°C / s or less within the temperature range from 0°C to the annealing temperature is effective. If the heating rate from 400°C to the annealing temperature exceeds 20°C / s, precipitation will occur during the hot rolling stage. Because there is insufficient time for the carbides to be redissolved, there is a possibility that coarse carbides may remain. This could potentially reduce the impact energy absorption capacity of the final hot-formed component. Therefore, it is effective for the heating rate from 400°C to the annealing temperature to be 20°C / s or less. Yes. The above heating rate is more effective when it is 18°C / s or less, and 15°C / s or less. It is even more effective if there is a certain condition. On the other hand, in the present invention, the above heating rate is 20°C / s or less. If so, the effect that the present invention aims to achieve can be obtained, therefore, the lower limit of the heating rate is not particularly limited. It is not limited to this. However, considering annealing productivity, the above heating rate should be 0.5°C / s or higher. Even if it is 1°C / s or more, and more effectively 1.5°C / s or more Good. On the other hand, in the present invention, in the temperature range from the cold rolling temperature to less than 400°C, the heating rate This is not particularly limited to this. This is because even if the heating rate is controlled, the effect on the redissolution of carbides is... Because it is small.

[0071] It is effective to anneal the heated cold-rolled steel sheet at an annealing temperature of 740-860°C. If the annealing temperature is below 740°C, sufficient recrystallization of the cold-rolled microstructure will not occur. Therefore, the plate shape may become defective, or the strength after plating may become too high, during the blanking process. This can induce mold wear. On the other hand, if the annealing temperature exceeds 860°C, annealing A problem arises where surface oxides such as Si and Mn form during the process, resulting in a defective plated surface. Therefore, the above annealing temperature is effective when it is between 740 and 860°C. A lower limit of 750°C is more effective, and 760°C is even more effective. Yes. The upper limit of the above annealing temperature is more effective at 850°C, and 840°C is also effective. This is even more effective.

[0072] It is effective to use a non-oxidizing atmosphere during the annealing process described above. For example, a hydrogen-nitrogen atmosphere. A mixed gas can be used, and in this case, the dew point temperature of the atmospheric gas is ) can be -70 to -30°C. In order for the above dew point temperature to be below -70°C However, this poses a problem as it requires additional equipment for control, which increases manufacturing costs. If the temperature exceeds -30°C, an excess of annealing oxide is formed on the surface of the steel sheet during annealing, and the unplated surface... This could cause any defects. Therefore, the atmospheric gas during the continuous annealing process described above An effective dew point temperature is -70 to -30°C. A lower limit of the dew point temperature for the atmospheric gas is -65°C, which is more effective than -60°C. This is even more effective. The upper limit of the dew point temperature of the above atmospheric gas is -35°C. It is more effective, and -40°C is even more effective.

[0073] The above annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C / s or more. (Annealing cooling) is performed. If the cooling rate is less than 1°C / s, a large amount of coarse carbides will be formed. The impact energy absorption capacity of the final hot-formed member may be reduced. Therefore, the above cooling rate should be 1°C / s or higher for it to be effective. A temperature of ℃ / s or higher is more effective, and a temperature of 2℃ / s or higher is even more effective. The above cooling rate does not have any particular upper limit. However, the upper limit of the cooling rate is limited. From this perspective, the above cooling rate may be 50°C / s or less, and more effectively 45°C / s or less. Furthermore, it may be even more effective if the temperature is below 40°C / s.

[0074] Plating The above annealed cold-rolled steel sheet can be further plated. In this invention, the type of plating While there are no particular limitations on the type and method, an example of Al-based plating will be described. First, the annealed cold-rolled steel sheet is cooled and immersed in an aluminum-based plating bath to apply an aluminum-based plating. A layer is formed. The composition of the Al-based plating bath and the plating conditions are not particularly limited.

[0075] However, as a non-limiting example, the composition of the plating bath is, in weight percent, Si: 6-12%, Fe : May contain 1-4%, with the remainder being Al and other unavoidable impurities, and the plating amount is determined by the technology. The standard single-sided guideline typically applied in the surgical field is 30-130 g / m². 2 It can be. If the Si content in the plating bath composition is less than 6% by weight, the plating bath temperature will rise excessively. It has the disadvantage of degrading the equipment, and if it exceeds 12% by weight, it excessively delays alloying. This has the disadvantage of requiring a longer heating time for hot forming. Fe content If the amount is less than 1% by weight, the plating adhesion and spot weldability may be poor, and 4% by weight If the percentage exceeds %, excessive dross formation occurs in the plating bath, leading to poor surface quality. This can happen. The plating adhesion amount is 30g / m² on one side. 2 If less than the desired hot-formed part This may make it difficult to ensure the corrosion resistance of the material, at 130g / m 2 If it exceeds this, excessive The amount of plating applied increases not only manufacturing costs, but also the amount of plating on the steel sheet becomes uniform. It may not be easy to plate the entire width and length of the coil.

[0076] On the other hand, according to another embodiment of the present invention, as described above, continuous annealing and Plating can be performed, but the plating should be done immediately after pickling on the cooled hot-rolled steel sheet. It is also possible.

[0077] Next, one embodiment of the hot-formed member of the present invention will be described in detail. The components can be manufactured by hot-press forming the hot-forming steel material described above.

[0078] The above hot-formed member includes a base material and a plating layer formed on the base material, and the following [related] The surface carbon concentration index after hot forming, as defined by Equation 2, is... An egregation factor of 0.1 or higher is considered effective.

[0079] [Relationship 2] C (peak、HPF後) / C (nom、HPF後 )≧0.1 (In relational equation 2, C (peak、HPF後) This is at a point on the surface where the plating layer thickness is 1 / 3 of the way through. From the direction of the base steel sheet, the first carbon peak that appears in the carbon profile, according to the GDS analysis results. (Peak) is the highest carbon value, C (nom、HPF後) Nominal of steel (Nom (This is the carbon value.)

[0080] The above hot-formed member has a surface carb concentration index (Surface carb) after hot forming. The segregation factor (SPF) is 0.1 or higher, and the surface quality is excellent. And fatigue resistance can be ensured. Table after hot forming as defined in [Relational Equation 2] above. Surface carbon segregation factor It is effective if r) does not exceed 1.0.

[0081] The surface carbonization concentration index before hot forming, as defined by relational equation 1 above, is determined by the heat defined by relational equation 2. It is effective if it is larger than the surface carbonization concentration index after interforming ([Relationship 1]>[Relationship Equation 2]).

[0082] On the other hand, C (nom、HPF後) This is the nominal carbon value of steel, and Therefore, it can be considered that there is almost no difference before and after hot forming. In this case, without distinguishing between before and after HPF, C nom It is sometimes written as follows.

[0083] The base material of the hot-formed member described above satisfies the alloy composition described above. On the other hand, the microstructure of the base material is Rutensite monophase structure or mixed structure containing martensite and bainite at 40% area or less. It can have the above martensite, which is effective in securing the strength targeted by the present invention. Because of this structure, the microstructure of the above-mentioned component may be a single-phase martensite structure. Although bainite is a slightly weaker tissue than martensite, it forms martensite bases. Because it is a tissue that is advantageous for ensuring strength without significantly reducing flexibility during internal formation, In this invention, the mixed structure includes bainite at a concentration of 40% or less in area, along with the martensite. It is also possible to do so. However, if the above bainite fraction exceeds 40 area%, the present invention Therefore, it may be difficult to achieve the target intensity.

[0084] On the other hand, the above microstructure contains ferrite at a rate of 10% or less by area and residual austenite at a rate of 5% or less. It may further include one or more of the above ferrite and retained austenite. This is something that can inevitably be formed during the manufacturing process. The above ferrite structure exceeds 10 area percent. In such cases, not only is the strength reduced, but the bending properties may also be significantly inferior, as described above. If the austenite structure exceeds 5% area, the strength may decrease, or the atmosphere during hot forming may be affected. An increase in hydrogen inflow from ambient gases could lead to a higher likelihood of hydrogen embrittlement occurring.

[0085] The plating layer of the hot-formed member described above is the plating layer of the steel material described above, after hot forming. This refers to the plating layer obtained.

[0086] The above hot-formed member may have an improvement in fatigue limit of 5% or more. The above improvement in fatigue limit means Because pre-alloying was not performed, the improvement rate of the fatigue limit for materials without carbon concentration on the surface was This means that the above improvement in fatigue limits can be confirmed by tensile-compression fatigue tests. If the working limit is improved by 5% or more, durability can be improved even with similar tensile properties.

[0087] Next, one embodiment of the method for manufacturing the hot-formed member of the present invention will be described in detail. The manufacturing method described below is only one of all possible embodiments, and the above heat This means that intermolding members should not necessarily be manufactured by any other method. There isn't one.

[0088] Prepare the hot-formed steel material described above or the hot-formed steel material manufactured by the method described above, and use it A blank is manufactured, and the above blank is heated to a temperature above the austenite single-phase region, more specifically Ac3 After heating to a temperature of ~980°C, hold for 1 to 1000 seconds.

[0089] If the blank heating temperature is below the Ac3 temperature, the presence of untransformed ferrite will result in a predetermined temperature. It may be difficult to ensure sufficient strength. On the other hand, if the heating temperature exceeds 980°C, the material table Excessive oxides may form on the surface, potentially making it difficult to ensure spot weldability. Therefore, the above blank heating temperature is effective when it is between 3°C and 980°C. The lower limit of the heating temperature is more effective at Ac3+5°C, and Ac3+10°C. This is even more effective. The upper limit of the blank heating temperature mentioned above is 970°C, which is more effective. It is effective, and 960°C is even more effective.

[0090] If the holding time is less than 1 second, the temperature will not be uniform throughout the entire blank, and different parts will be affected. This can induce differences in material properties, and if the holding time exceeds 1000 seconds, it can result in excessive heating temperature and other problems. In this way, excessive oxides may be generated on the surface of the component, making it difficult to ensure spot weldability. Yes. Therefore, the above holding time is effective when it is between 1 and 1000 seconds. A lower limit of 30 seconds is more effective, and 60 seconds is even more effective. Yes. The upper limit of the above holding time is more effective at 900 seconds, and 800 seconds is also effective. This is even more effective.

[0091] Afterward, the heated and held blank is hot-formed and then cooled to room temperature (molding cooling). ) and finally manufacture the hot-formed member. The specific conditions for the hot forming described above are not particularly important. Without limitation, any hot forming method commonly known in the art to which this invention belongs may be applied as is. It can be used. A preferred example is a mold cooling method. [Examples]

[0092] Next, embodiments of the present invention will be described.

[0093] The following examples are intended for use by persons with ordinary skill in the art to which the present invention pertains. It goes without saying that various modifications are possible within the scope of the invention. See below. The embodiments described below are for the purpose of understanding the present invention, and the scope of the present invention is not limited to the embodiments described below. It shall not be fixedly defined, and shall not be limited to the claims described below, but also to equivalent terms. It should be stipulated that way.

[0094] (Examples) The following composition (by weight %) is shown in Table 1 below, and the thickness is 100 mm. The steel slab was manufactured by vacuum melting.

[0095] The above steel slab is heated to 1250°C, and then hot-rolled at a finish hot-rolling temperature of 900°C. After being wound at a winding temperature of 640°C, a hot-rolled steel sheet with a final thickness of 2.5 mm was manufactured. After pickling the rolled steel sheet, cold rolling was performed at a cold reduction ratio of 45% to produce cold-rolled steel sheet. 5 After annealing to a normal annealing temperature of 780°C under a % hydrogen-95% nitrogen atmosphere, the above After cooling the cold-rolled steel sheet, it was plated with an aluminum-based plating.

[0096] At this time, the composition of the Al-based plating bath is Al-9%Si-2%Fe and the remainder is unavoidable impurities. It is composed of 70 g / m² of plating adhesion on one side. 2 That's what I decided.

[0097] The surface layer of the plated steel sheet is subjected to a box annealing heat treatment for carbon enrichment, and the box annealing heat treatment Depending on the type of process, it can be done in one step or two steps. Box annealing was performed using the formula, and the specific conditions are described in Table 2 below.

[0098] [Table 1]

[0099] [Table 2]

[0100] In Table 2 above, H1: heating rate for one-stage box annealing, T1: target temperature for one-stage box annealing. Degrees, t1: holding time for one stage of box annealing, H2-1: heating rate for one section of two stages of box annealing, T2-1: Target temperature for one section of two-stage box annealing, t2-1: Maintenance temperature for one section of two-stage box annealing Holding time, H2-2: heating rate in two sections of two-stage box annealing, T2-2: two sections of two-stage box annealing The target temperature for each section, and t2-2: refers to the holding time for the two sections of the two-stage box annealing process.

[0101] After the steel plates manufactured in this manner are made into blanks, they are hot-formed using a hot-forming die. A hot-formed component was manufactured by this process. At this time, the heating temperature of the blank was 900°C. The holding time is 5 minutes, and the transport time from the heating furnace to molding is 10 seconds in all cases, and the same applies. did.

[0102] Surface carbon concentration index of the above hot-formed steel material and hot-formed member (Surface carbon The segregation factor was measured and is shown in Table 3 below. The above surface carbonization The concentration index was determined by measuring the carbon distribution in the depth direction using a GDS analyzer, and a detailed analysis is shown in Figure 1. The method was shown. Of the GDS carbon distribution from one-third of the surface layer including the plating layer, the first carbon The highest carbon content of the elementary peak is C peak C nom This represents the average carbon content for each type of steel. On the other hand, in the graph in Figure 1(b), the solid line represents the carbon distribution before HPF, and the dotted line represents the carbon distribution after HPF. This demonstrates the following.

[0103] [Table 3]

[0104] In Table 3 above, relational equations 1 and 2 are, respectively, C (peak、HPF前) / C (nom、HPF前) and C (peak、HPF後 ) / C (nom 、HPF後) Calculation It is a value.

[0105] On the other hand, whiteness was measured using a colorimeter with tape that had been attached to the surface of the material and then peeled off. This is the value obtained. In the case of fatigue limits, it is derived by repeatedly performing compression-tensile tests under specific load conditions. This value represents the improved fatigue limit for materials that have not undergone pre-alloying heat treatment and have no surface concentration. The improvement rate is expressed as the fatigue limit improvement percentage (%). The edge portion of the pre-alloyed material was subjected to electron The edges were observed under a microscope to determine whether or not they were melted, and this was expressed as O / X.

[0106] As can be seen from Tables 1 and 2 above, all of the alloy compositions and box annealing conditions proposed in this invention are met. In addition, it was confirmed that excellent surface quality and fatigue characteristics can be ensured in the cases of Invention Examples 1 to 7.

[0107] Comparative Examples 1, 5, 10, and 15 satisfy the alloy composition proposed in the present invention, but box annealing heat This refers to products that have not undergone any treatment, and therefore, not only is the alloying of the plating layer confirmed, but the surface carbon concentration index is also confirmed. I was unable to keep it.

[0108] Comparative Examples 2 and 12 show that the heating rate in the box annealing process is the heating rate proposed in the present invention. Although present, the heat treatment temperature is less than 500°C, resulting in insufficient alloying and the surface carbon of the steel material before hot forming. The concentration index fell outside the scope of the present invention, resulting in only a slight improvement in the fatigue limit. Figure 2 (a) and (b) respectively show the cross-section of the plating layer after box annealing heat treatment in Invention Example 1 and Comparative Example 2. This is a photograph of the surface. Looking at Figure 2(b) above, in Comparative Example 2, the holding time is the same as in the present invention. Despite meeting the stated conditions, the target temperature is not met, therefore sufficient Goldification was not achieved. On the other hand, in the case of Invention Example 1, all the pre-alloying heat treatment conditions were met. This indicates that the alloying process was completed after the alloying heat treatment.

[0109] Comparative Example 3 is a material that has undergone a one-stage box annealing heat treatment, and the flow rate and circulation of the purge gas The fan speed is sufficient, but the heating rate is less than 20°C / hr, which is causing surface oxide formation. It was confirmed that the whiteness was inferior, with a value of less than 60. On the other hand, in Comparative Example 11, the heating rate was the same as in the present invention. Beyond the specified range, the surface quality deteriorated due to the melting of the plating layer caused by overheating at the edges.

[0110] Comparative Examples 7 and 14 applied a two-stage box annealing heat treatment, and one section of the two-stage heat treatment The heating rate exceeds the range of the present invention, and excessive heat treatment causes overheating of the edge portion. The surface quality deteriorated due to the melting phenomenon of the plating layer. Figures 3(a) and (b) show, respectively, These are photographs of Invention Example 2 and Comparative Example 7 observed with an optical microscope, compared to Invention Example 2 in Figure 3(a). In Comparative Example 7 in Figure 3(b), the plating layer melted at the edge and then solidified, and the surface It was confirmed that the quality was not uniform (in the box portion).

[0111] Comparative Examples 8 and 13 each had a two-stage box annealing heat treatment process, with the heating rate in two sections being different. This invention exceeds or falls short of the scope of the present invention, and each involves the plating layer at the edge portion. It was confirmed that melting occurred or that the surface quality deteriorated, with a whiteness of less than 60.

[0112] Although Comparative Examples 4 and 6 meet the requirements for heating rate and holding time, the flow rate of the purge gas is The invention did not meet the scope of the present invention and could not ensure sufficient surface whiteness. Comparative Examples 9 and 16 show that during box annealing, the operation of the circulation fan inside the furnace was less than 10 rpm. Due to uneven heating of the furnace, the edge area may overheat, causing the plating layer to melt and affect the surface of the product. A decline in quality was confirmed.

Claims

1. The material includes a base steel sheet and a plating layer formed on the base steel sheet. The aforementioned base steel sheet contains, by weight, C: 0.04 to 0.45%, Si: 1.5% or less (0%). (excluding), Mn: 0.2-2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0 .. 01 to 0.1%, Cr: 0.01 to 5.0%, N: 0.02% or less, balance Fe and impossibility Contains impurities, A steel material for hot forming that satisfies the following [Relationship Formula 1]. [Relationship 1] Carbon concentration index before hot forming: C （ｐｅａｋ、ＨＰＦ前） / C （ｎｏｍ、ＨＰＦ前） ≥ 1. ５ (In relational equation 1, C （ｐｅａｋ、ＨＰＦ前） This is at a point on the surface where the plating layer thickness is 1 / 3 of the way through. From the direction of the base steel sheet, the GDS analysis results show that the first carbon particle appearing in the carbon profile is... The highest carbon value in Peak is C （ｎｏｍ、ＨＰＦ前） is a steel nominal (N (This is the carbon value.)

2. The aforementioned base steel sheet contains Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, T The claim 1 further comprises one or more of i: 0.1% or less and B: 0.01% or less. Steel material for hot forming.

3. The microstructure of the aforementioned base steel sheet contains 50-90% ferrite by area fraction, and pearlite Contains one or more of the following: 30% or less, bainite 20% or less, and martensite 20% or less. The steel material for hot forming according to claim 1.

4. The hot-forming steel material according to claim 1, wherein the whiteness of the hot-forming steel material is 60 or higher.

5. In weight percent, C: 0.04-0.45%, Si: 1.5% or less (excluding 0%), Mn: 0 .. 2-2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01-0.1% Steel containing Cr: 0.01-5.0%, N: 0.02% or less, the remainder being Fe and unavoidable impurities. The step of obtaining plated steel sheet using a slab, The process includes the step of box annealing the plated steel sheet in a one-step manner as described in [Box Annealing Condition 1] below, and the heat A method for manufacturing steel materials for interforming. [Box annealing condition 1] Temperature range (T1): 500 to 800°C Holding time (t1): 1 minute or more (this is the holding time at the target temperature) Heating rate (H1): 20–160°C / hr (This is the heating rate to reach the target temperature.)

6. In weight percent, C: 0.04-0.45%, Si: 1.5% or less (excluding 0%), Mn: 0 .. 2-2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01-0.1% Steel containing Cr: 0.01-5.0%, N: 0.02% or less, the remainder being Fe and unavoidable impurities. The step of obtaining plated steel sheet using a slab, The process includes the step of box annealing the plated steel sheet in the two-step method described in [Box Annealing Condition 2] below, and the heat A method for manufacturing steel materials for interforming. [Box annealing condition 2] Temperature range for one interval (T2-1): 500 to 780°C Holding time for one section (t2-1): 1 minute or more (this is the holding time at the target temperature). Heating rate per section (H2-1): 20-160°C / hr (This is the heating rate to reach the target temperature.) ） Temperature range for two intervals (T2-2): 600-800°C Holding time for the two sections (t2-2): 50 minutes or more (this is the holding time at the target temperature). Heating rate in two sections (H2-2): 0.25 to 160°C / hr (at the heating rate up to the target temperature) be)

7. The steel slab contains Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, T Claim 5 or 6 further comprises one or more of i: 0.1% or less, and B: 0.01% or less. A method for manufacturing hot-forming steel materials as described above.

8. During the aforementioned box annealing, the purge gas inside the furnace is hydrogen (H ２ ), nitrogen (N ２ ) and these mixed gas It is one of the following, and the purge amount is 0.1 to 100 m ３ Claim 5 is / hr The method for manufacturing hot-forming steel materials as described in 6.

9. The circulation fan can be operated during the annealing of the box, and the operating rate of the circulation fan is 10 rpm. A method for manufacturing hot-forming steel materials according to claim 5 or 6, wherein the length is 1 m or more.

10. The step of obtaining the aforementioned plated steel sheet is: The steps include heating the steel slab to 1050-1300°C, The step of obtaining a hot-rolled steel sheet by finishing hot-rolling the heated steel slab at 800 to 950°C. and, The steps include winding the hot-rolled steel sheet at 500 to 700°C, The aforementioned wound hot-rolled steel sheet is cooled from the winding temperature to 400°C at a cooling rate of 10°C / hr or more. The cooling stage, The steps include: cold rolling the cooled hot-rolled steel sheet at a reduction ratio of 30-80% to obtain a cold-rolled steel sheet; 、 The cold-rolled steel sheet is heated in a temperature range from 400°C to the annealing temperature at a rate of 20°C / s or less. The stage, The steps include annealing the heated cold-rolled steel sheet at an annealing temperature of 740 to 860°C, The annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C / s or more. The stage, The hot-forming steel material according to claim 5 or 6, comprising the step of plating after annealing. Manufacturing method.

11. During the aforementioned annealing process, the dew point temperature of the atmospheric gas is -70 to -30°C. The method for manufacturing hot-forming steel materials according to claim 10.

12. The aforementioned plating consists of, by weight %, Si: 6-12%, Fe: 1-4%, and the remainder being Al and unavoidable The aluminum plating layer is formed by immersion in an Al-based plating bath containing pure material, as described in claim 10. A method for manufacturing hot-forming steel materials.

13. In weight percent, C: 0.04-0.45%, Si: 1.5% or less (excluding 0%), Mn: 0 .. 2-2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01-0.1% The base material contains Cr: 0.01-5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities. The material and the plating layer formed on the base material, A hot-formed member that satisfies the conditions of the following [Relational Equation 2]. [Relationship Equation 2] C （ｐｅａｋ、ＨＰＦ後） / C （ｎｏｍ、ＨＰＦ後） ≧0.1 (In relational equation 2, C （ｐｅａｋ、ＨＰＦ後） This is at a point on the surface where the plating layer thickness is 1 / 3 of the way through. From the base steel sheet towards the surface, the GDS analysis results show the first carbon peak appearing in the carbon profile. (Peak) is the highest carbon value, C （ｎｏｍ、ＨＰＦ後） Nominal of steel (Nom (This is the carbon value.)

14. The aforementioned base material contains Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, Ti: The heat according to claim 13, further comprising one or more of B: 0.1% or less and B: 0.01% or less. Intermolding member.

15. The microstructure of the hot-formed member is a single-phase martensite structure or a martensite structure with 40 areas The hot-formable member according to claim 13, having a mixed structure containing % or less of bainite.

16. The hot-formed member according to claim 13, wherein the value of [relational formula 2] is 1.0 or less.

17. The improvement in the fatigue limit of the hot-formed member is 5% or more, as described in claim 13. Material.

18. The hot-formed member is made using the hot-formed steel material described in any one of claims 1 to 4. A hot-formed member as described in claim 13, manufactured.

19. A step of obtaining a blank using a hot-forming steel material according to any one of claims 1 to 4. 、 The blank is heated to a temperature of Ac3 to 980°C, and then held for 1 to 1000 seconds. and, The process includes the step of hot forming the heated and held blank and then cooling it. A method for manufacturing a shaped member.

20. The method for manufacturing a hot-formed member according to claim 19, wherein the cooling is performed by a mold cooling method.

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